1. Field of the Invention
The present invention relates to rotating electric machines such as electric generators and electric motors having a permanent magnet.
2. Description of the Related Art
Rotating electric machine apparatuses, such as an electric generator for generating electric power electromagnetically by relative rotation between a permanent magnet and an armature, or an electric motor for generating relative rotation between a permanent magnet and an armature by interaction between the permanent magnet and a magnetic field generated by current supplied to the armature, are excellent in energy efficiency and have been widely used routinely along with technical progress of permanent magnets. However, in both electric motors and electric generators, optimum power is not always obtained in a wide rotational speed range because of constant magnetic field strength from the field magnet.
In the case of the electric motor, the control thereof becomes difficult in a high-speed rotational region because the back electromotive force (power generation voltage) becomes too high, and therefore, various methods for weakening the field strength as field-weakening control have been proposed. Moreover, in the case of the electric generator, a constant-voltage electric generator by only field current control or a constant-voltage circuit in which the power generation voltage is made to be constant by a semiconductor has been exclusively used so that the power generation voltage is made to be a predetermined level in a wide rotational-speed range. In the electric motor, field-weakening control by current phase control has been widely adopted, but energy loss is large because current flows that does not directly contribute to the rotation. When current excitation for the control is used with a permanent magnet excitation, the structure of the rotating electric machine becomes complex and additionally energy loss is involved. Furthermore, in the case of the electric generator, there has been a problem that cost of constant-voltage electronic circuit with a large electric power is large. Under such a circumstance, measures for reducing the cost of the entire apparatus while binding the electronic-circuit control to a minimum by devising the structure of the rotating electric machine apparatus have been required for a long time, and various proposals have been made.
In U.S. Pat. No. 3,713,015, there is described an alternating current generator in which a permanent magnet rotor is divided into two and the two permanent magnet rotors are relatively displaced in the circumferential direction to effectively control the field strength. There is an advantage that the energy loss for the control is small because the relative displacement can be maintained mechanically, but there is a disadvantage that eddy-current loss is large in a high-speed rotational region because the amount of the magnetic flux flowing into the armature is constant. In Japanese Unexamined Patent Publication (Kokai) No. 2004-320864 and No. 2004-328944, there are described apparatuses for controlling a magnetic field strength by changing magnetic resistance in a magnetic circuit including the field magnet. Furthermore, in U.S. Pat. No. 4,885,493, Japanese Unexamined Patent Publication (Kokai) No. 2004-357357 and No. 2006-246662, apparatuses for making the field magnet short have been described. In general, when the magnetic circuit including the magnet has a movable part, there is a magnetic force of displacing the movable part to the direction in which the magnetic flux quantity flowing through the magnetic circuit becomes larger (the direction in which the magnetic resistance becomes smaller). The field magnet is a source for generating force or generating power in the rotating electric machine apparatus, and the magnetic force thereof is proportional to the output of the rotating electric machine in the proposed example of the rotating electric machine apparatus for controlling the magnetic resistance of the magnetic circuit or for short-circuiting the field magnet by mechanical displacement. A large force is required for the displacement control of the mechanism and vibration or hunting of the members is caused to make it difficult to perform the accurate control. Furthermore, a large-power actuator, a control mechanism involving excessive mechanical strength, and so forth are required, and therefore, realization of the apparatus involves difficulty.
The contents of U.S. Pat. Nos. 3,713,015 and 4,885,493, Japanese Unexamined Patent Publication Nos. 2004-320864, 2004-328944, 2004-357357, and No. 2006-246662 are incorporated herein by reference in their entirety.